A thin-film forming process for forming a CVD thin film such as the silicon nitride film (a Si3N4 film) on a substrate such as a semiconductor wafer using a thermal chemical vapor deposition (“CVD”) technique is one of processes of manufacturing a semiconductor device. The thin-film forming process using thermal CVD technique is performed by supplying a processing gas into a processing chamber having the substrate loaded therein. While the thin-film forming process is aimed at forming the thin film on a surface of the substrate, an undesirable deposit including the thin film is also adhered to regions other than the surface of the substrate, for example, to an inner wall of a reaction tube constituting the processing chamber. The deposit is accumulatively adhered each time the thin-film forming process is performed, and delaminated when the deposit reaches a predetermined thickness or more, thereby generating foreign matters (particles) in the processing chamber. Accordingly, whenever the deposit reaches the predetermined thickness, it is necessary to clean the inside of the processing chamber and members disposed in the processing chamber by removing the deposit.
The configuration of the typical CVD thin-film forming apparatus for a semiconductor will be described with reference to FIG. 1. The thin-film forming apparatus may include a reaction tube 103 including a film-forming chamber (a processing chamber 101) configured to process substrates 100, a boat 102 configured to hold the substrate 100 in the film-forming chamber 101 in multiple stages in a horizontal posture, a heating source 104 disposed around the reaction tube 103, a processing gas supply line 105 through which a processing gas for forming the CVD thin film is supplied into the film-forming chamber 101, a cleaning gas supply line 107a through which an NF3 gas serving as a cleaning gas for removing a deposit via an etching process is supplied into the film-forming chamber 101, an additional gas supply line 107b through which an NO gas to be added to the cleaning gas is supplied, and an exhaust line 108 at which a pressure adjustment valve 106 for adjusting an inner pressure of the film-forming chamber 101 and a vacuum pump 109 are installed sequentially from an upstream side. The reaction tube 103 and the boat 102 are made of quartz (SiO2).
Hereinafter, the thin-film forming process using the above-described thin-film forming apparatus will be described. First, the boat 102 holding the substrates 100 is loaded into the film-forming chamber 101. Thereafter, a surface of each of the substrates 100 is heated by the heating source 104 to a predetermined temperature. Afterwards, the processing gas is supplied through the processing gas supply line 105 into the film-forming chamber 101 while exhausting an inside of the film-forming chamber 101 through the exhaust line 108, and a thin film is then formed on the substrates 100 by a CVD reaction. A pressure of the film-forming chamber 101 is adjusted by the pressure adjustment valve 106 installed at the exhaust line 108 in order to maintain the pressure of the film-forming chamber 101 at a predetermined pressure. When the thin film having a predetermined thickness is formed on the substrates 100, the supply of the processing gas from the processing gas supply line 105 is stopped. Thereafter, the substrates 100 having the thin film formed thereon are cooled to a predetermined temperature, and the boat 102 is then unloaded from the film-forming chamber 101.
The above-described thin-film forming process is originally for forming the thin film on the substrate 100. However, when the thin film is formed on the substrate 100, a deposit including the thin film is actually adhered to surfaces of members such as an inner wall of the reaction tube 103 that constitutes the film-forming chamber 101 and the boat 102 each time the above-described thin-film forming process is performed. When the deposit reaches a predetermined thickness or more, the deposit is delaminated or dropped, thereby generating foreign matters on the substrate 100. Therefore, the deposit should be removed whenever the deposit reached the predetermined thickness.
Conventionally, a wet cleaning process including separating the reaction tube 103 from the substrate processing apparatus, dipping the reaction tube 103 in a cleaning solution including an HF solution, and removing the deposit by wet etching has been mainly adopted as a method for removing the deposit.
In recent years, however, a dry cleaning process wherein the reaction tube is not separated is increasingly used. The dry cleaning process includes directly supplying a gas (hereinafter, referred to as a “cleaning gas”) containing fluorine (F) atoms and a dilution gas into a processing chamber and removing a silicon-based deposit such as a Si3N4 film by etching by controlling a temperature, a pressure, and a gas flow rate in the processing chamber. In addition, a technique of adding a gas containing oxygen (O) atoms to the cleaning gas is known as a means for improving the performance (i.e., etching rate) of the cleaning process (for example, refer to Japanese Patent Laid-Open Publication Nos. HEI 10-303186 and 2005-101583).
Furthermore, a technique using a FNO gas as the cleaning gas is also known (for example, Japanese Patent Laid-Open Publication No. 2003-144905.
A nitrogen monoxide (NO) gas is popular as the gas containing oxygen atoms to be added to the cleaning gas. However, when the NO gas is added to the cleaning gas, processing conditions such as a temperature and a pressure need to be adjusted to a high temperature and a high pressure or a cleaning rate is reduced. For example, when a nitrogen trifluoride (NF3) gas is used as the cleaning gas, the processing chamber must be maintained under the condition of the high temperature and the high pressure in order to cause a sufficient reaction of the NF3 gas with the NO gas due to a low reactivity of the NF3 gas with the NO gas. Moreover, when a fluorine (F2) gas is used as the cleaning gas, the F2 gas excessively reacts with the NO gas due to a high reactivity of the F2 gas with the NO gas, resulting in a degradation of the cleaning rate. That is, the addition of the NO gas to the cleaning gas results in a difficulty in handling and a degradation of a controllability of the cleaning performance.
In addition, when the FNO gas is used as the cleaning gas, the cleaning process is not performed in some cases. While the FNO gas added to the cleaning gas accelerates an etching reaction by the cleaning gas, using only the FNO gas does not facilitate an etching process due to a low etching rate. Furthermore, FNO is not commercialized as a gas presently, and an immediate adoption of the FNO gas as the cleaning gas is difficult. Furthermore, the above-described problems have been only discovered in latest research conducted by the inventors.
Hereinafter, the dry cleaning method will be briefly described. First, the empty boat 102 having the deposit adhered thereon is loaded into the reaction tube 103, namely, into the film-forming chamber 101 also having the deposit adhered thereon. Thereafter, the inside of the film-forming chamber 101 is heated by the heating source 104 to a predetermined temperature. Next, the NF3 gas is supplied through the cleaning gas supply line 107a into the film-forming chamber 101 while exhausting the inside of the film-forming chamber 101 through the exhaust line 108, and the deposit adhered to the inside of the film-forming chamber 101, namely, the inner wall of the reaction tube 103 or the surface of the boat 102, is removed due to an etching reaction of active species generated by a decomposition of the cleaning gas with the deposit. Here, the NO gas is supplied from the additional gas supply line 107b and is added to the NF3 gas supplied into the film-forming chamber 101 to improve the etching rate. Further, the pressure in the film-forming chamber 101 is adjusted by the pressure adjustment valve 106 installed at the exhaust line 108 in order to maintain the pressure of the film-forming chamber 101 at the predetermined pressure. When the deposit is removed from the film-forming chamber 101, the supply of the cleaning gas from the cleaning gas supply line 107 is stopped. Thereafter, a seasoning process is performed in the film-forming chamber 101. That is, the processing gas is supplied into the film-forming chamber 101 with the substrate 100 unloaded therefrom, and the thin film is formed (pre-coated) on the inner wall of the reaction tube 103 or the surface of the boat 102 in the film-forming chamber 101 so that the film-forming chamber 101 can be restored to a state in which the film forming process may be performed.
In addition to the NF3 gas as the nitrogen fluoride gas, a F2 gas may be used as the cleaning gas. However, as described above, when the NF3 gas is used as the cleaning gas, the processing chamber must be maintained under the condition of the high temperature, for example 600° C. and the high pressure in order to cause the sufficient reaction of the NF3 gas with the NO gas due to the low reactivity of the NF3 gas with the NO gas. Moreover, when the F2 gas is used as the cleaning gas, the F2 gas excessively reacts with the NO gas due to a high reactivity of the F2 gas with the NO gas, resulting in a degradation of the cleaning rate. That is, the addition of the NO gas to the cleaning gas results in the difficulty in handling and the degradation of the controllability of the cleaning performance.
In addition, a technique wherein only the FNO gas is used as the cleaning gas is also known. However, when only the FNO gas is used as the cleaning gas, the cleaning process is not performed in some cases according to the research by the inventors. While the FNO gas added to the cleaning gas accelerates the etching reaction by the cleaning gas, using only the FNO gas does not facilitate the etching process due to the low etching rate. Furthermore, FNO is not commercialized as the gas presently, and the immediate adoption of the FNO gas as the cleaning gas is difficult.